1. Field of the Invention
The present invention relates to a capacitive high voltage generator.
2. Description of the Related Art
As is known, capacitive high voltage generators of the prior art exploit the many stages of which they are composed to supply voltages at outputs which are higher than the supply voltage.
For simplicity""s sake, FIG. 1 shows a capacitive single stage high voltage generator 50. The single stage 1 of the voltage generator 50 has an input terminal 2 receiving a timing signal CK and an output terminal 3 supplying an output voltage V0.
Stage 1 comprises a buffer 4, of an inverting type, a boost capacitor 5, a first and a second diode 6, 7, and a filter capacitor 8. In detail, the buffer 4 has a first terminal 9 connected to a first supply line 10 set at a first reference potential V1, a second terminal 11 connected to a ground terminal GND and an input terminal connected to an input terminal 2 of the stage 1. The boost capacitor 5 has a first terminal connected to an output terminal 12 of the buffer 4 and a second terminal connected to an intermediate node 13. The first diode 6 has an anode terminal 14 connected to a second supply line 15 set at a second potential V2 and a cathode terminal connected to the intermediate node 13. The second diode 7 has an anode terminal connected to the intermediate node 13 and a cathode terminal connected to the output terminal 3 of the stage 1. The filter capacitor 8 has a first terminal connected to the output terminal 3 and a second terminal connected to the ground terminal GND.
When a higher output voltage is required, the high voltage generator 50 comprises several stages cascade-connected and structurally the same as stage 1 in FIG. 1. In particular, in the example shown in FIG. 2, the high voltage generator 50 comprises a first and a second stage 1a, 1b, the second stage 1b having the anode terminal 14b, of the corresponding first diode 6b, directly connected to the output terminal 3a of the first stage 1a.
If the high voltage generator 50 is formed by n stages and the first potential V1 is the same as the second potential V2, the output voltage V0 is equal to:
V0=(n+1)V1xe2x88x922nVD
where VD is the voltage present across each diode.
As it possesses a silicon technology that is able to support high voltages, the high voltage generator 50 may be advantageously realized as shown in FIG. 3 in which each stage 1a, 1b has the anode terminal 14a, 14b of the corresponding first diode 6a, 6b and the first terminal 9a, 9b of the corresponding buffer 4a, 4b set at the same reference potential.
In this case the output voltage V0 is equal to:
V0=2nV1+(2xe2x88x922n+1)VD
However, the high voltage generators of the prior art are poorly efficient when they are supplied with voltages of a few Volts (for example 1.8V) which are imposed by the use of more and more advanced submicrometric technologies.
The aim of the present invention is to provide a capacitive high voltage generator having greater efficiency than that which may be obtained with capacitive high voltage generators of the prior art and able to supply high output voltages even when starting from initial voltages of only a few Volts.
In one embodiment, the invention resides in a capacitive high voltage generator operable in response to input timing signals and connectable to first and second supply inputs and a reference potential. The generator includes first, second and third charging circuits. Each charging circuit has a control element, a switch element and a boost capacitor. The control element of the first and second charging circuits are configured to alternatively connect the boost capacitor thereof to the first supply input and to the reference potential in response to one of the input timing signals. The switch element is connected to the boost capacitor of each charging circuit with the junction thereof forming an output terminal of the charging circuit. The switch element of the first and second charging circuits is connected to the second supply input. The control element of the third charging circuit is configured to alternatively connect the boost capacitor thereof to the output terminal of the first charging circuit and to the reference potential in response to one of the input timing signals. The switch element of the third charging circuit is connected to the output terminal of the second charging circuit. The output of the third charging circuit is the output of the combination of the first, second and third charging circuits.
In this embodiment, the switch element of each of the first, second and third charging circuits may be a diode. Alternatively, the switch element of each of the first, second and third charging circuits may be a MOS transistor. Or, the switch element of the first, second and third charging circuits may perform a synchronous rectification. The control element of each of the first, second and third charging circuits may be an inverting buffer.
In one illustrated embodiment, each of the first, second and third charging circuits is configured for connection to first and second supply inputs and a reference potential. Each charging circuit includes a buffer element, a switch element and a boost capacitor. The boost capacitor has first and second terminals. The switch element has first and second terminals. The buffer element has a timing input to receive the input timing signal or the negative thereof, a first terminal, a second terminal connected to the reference potential, and an output connected to the first terminal of the boost capacitor. The charging circuit is configured to alternatively connect the first terminal of the boost capacitor to the first supply input and to the reference potential in response to the input timing signal. The second terminal of the switch element is connected to the second terminal of the boost capacitor with the junction thereof forming an output terminal of the charging circuit. The first terminal of the buffer element of the first and second charging circuits is connected to the first supply input. The first terminal of the switch elements of the first and second charging circuits is connected to the second supply input. The first terminal of the buffer element of the third charging circuit is connected to the output terminal of the first charging circuit. The first terminal of the switch element of the third charging circuit is connected to the output terminal of the second charging circuit. The output terminal of the third charging circuit is configured as the output of the combination of the first, second and third charging circuits.
Other features and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings.
According to the present invention a capacitive high voltage generator is provided.